The term “melt blown” refers to fibers or a mat formed by extruding a molten thermoplastic material (the “melt” or the “polymer melt”) through a plurality of fine orifices as molten filaments into converging flows of high-speed heated gas. This process is described more fully in U.S. Pat. No. 3,825,380; U.S. Pat. No. 3,849,241; and U.S. Pat. No. 4,889,476, all of which are incorporated by reference herein, as well as in numerous other publications. Generally speaking, the polymer is melted in an extruder and forced through a row of fine capillaries (also known as “orifices” or “nozzles” or “spinnerets”) to produce molten filaments. The orifices are drilled through the apex of a sharp angled metal structure called the “die tip.” Two adjacent parts known as “air plates” or “air blades” surround the die tip and define the gaps between them, which constitute the geometry of the “air knives”.
High pressure and temperature air or gas (known as the “primary air”) passes through the air knives. The pressure of the supplied primary air determines the blowing speed of the air knives. The air knives attenuate and agitate the molten filaments as they exit the orifices to reduce their diameters and to improve the molecular alignment of the polymer. By regulating the temperature and pressure of the primary air and those of the polymer melt, this arrangement is capable of producing fibers of different diameter sizes, from the sub-micron diameter range to macro fibers (with average diameters of greater than 40 microns). According to U.S. Pat. No. 3,825,380, the upper limit for the diameter of the melt blowing orifice is about 0.03 inches. Orifices of larger size may produce excessively large shots in the resulted nonwoven products. Melt-blown fibers are sufficiently continuous and self-bonding, when deposited onto a collecting surface.
One of the major uses of nonwoven fabrics or articles (often simply called “nonwovens”) is for gas and liquid filtration. Melt-blown (MB) nonwovens are particularly suitable for such uses, because their micro-sized fibers and pores can trap even microscopic particles while still allowing flow to pass through the article. Melt-blown nonwovens meet stringent filtration requirements better than other nonwovens can, including spun bonded, air-laid, wet-laid, carded, needle-punched, and spun-laced nonwovens, as well as most glass or wood fiber mats.
In filtration applications, both planar mats and cylindrical tubular filters are common. The latter functions especially well in situations with tight space, large flow volume, or high flow pressure. The early tubular filters were made by rolling up nonwoven sheets and then cutting into a desired length. This method has been found to have some significant drawbacks.
First, this manufacturing approach requires numerous steps and pieces of equipment, including cutting off and recycling the two ends of the rolled-up tube. These discrete steps cannot be integrated into a continuous process flow. Predictably, the equipment, material, and labor costs are high. Additionally, the layered filter tubes have lower material utilization efficiency than that of a one-piece nonwoven tube of the same weight.
Secondly, the resulting rolled-up filter has deficiencies. For example, the layered tube has poor rigidity, and often requires a hard inner core to help withstand flow pressure. Incorporating such a core requires additional equipment, material, and costs. Moreover, seams are required to secure the inner and outer edges to the tube body, which can be problematic for some applications. Also, variations in the nonwoven sheets (such as variations in weight, thickness, porosity, and the like) result in variations in the final tubular product.
Lastly, this method is not suitable for making tubular filters with density-gradient walls, which are more popular in the industry due to their better filtration efficiency and longer service life.
Therefore, the filtration industry has enthusiastically pursued methods and equipment for making one-piece melt-blown (MB) tubes that are coreless and seamless. In this approach, a tubular nonwoven system 1 includes a melt-blowing die 101 that blows molten fibers 103 directly onto a rotating mandrel roll 102 to form a porous tubular nonwoven 104, as shown generically in FIG. 1. The distance between the die tip and the mandrel roll influences the tightness of the laid fibrous body. As the tube reaches the desired outer diameter, it is continuously withdrawn from the mandrel by a puller device 107 (as indicated by dashed lines, more details of which will be provided below). Downstream of the puller device 107, a motor-driven cutting device 105 (e.g., a flying knife or saw) is used to cut the moving tube 104 into nonwoven cartridges 106 of a desired length. The results are better products with low capital, labor, and waste.
Examples of such efforts to produce tubular melt-blown nonwovens may be found in U.S. Pat. No. 4,112,159; U.S. Pat. No. 4,116,738; U.S. Pat. No. 4,847,125; U.S. Pat. No. 5,366,576; U.S. Pat. No. 5,409,642; U.S. Pat. No. 5,591,335l; U.S. Pat. No. 5,672,232; U.S. Pat. No. 6,391,200; and U.S. Pat. No. 6,736,274. Some of these exemplary devices use a single die, while others use multiple dies.
It is well understood that there are numerous, sometimes conflicting, requirements to develop a satisfactory means of pulling the rotating tube off the mandrel continuously and steadily. Some of these requirements include:
a. The puller device should be economical to build, easy and safe to use.
b. The position and movement of the puller device should permit the safe and free movement of the tube-cutting knife or saw.
c. The puller device should be able to operate over a broad range of speeds and inner and outer tube diameters with quick and simple adaptation only, because tubular products are routinely made in many diameters, lengths, and wall thicknesses.
d. All melt-blown devices have a fluctuating and generally declining output rate, due to the gradual clogging of the melt filter, contamination of the nozzle, and possible fluctuation in the voltage received by the device. Therefore, the puller device's speed should be self-correcting or at least easily adjustable to maintain product consistency.
e. Physical damage to the inside or outside of the tubular cartridges should be avoided to preserve their functionality. Crushing, cuts, tears, scratches, punctures, fluid channeling, or loose fibers adversely affect the performance of the tubular cartridges. Cuts to the inside wall may be particularly destructive, as they cause filtrate leakage.
f. The rate of fiber mass being removed from the mandrel must match the rate of fiber deposition onto the mandrel, so that the resulting tube will have consistent weight and diameter. Slippage among the mandrel, the tube, and the puller device cannot be tolerated.
g. When the puller device pulls the tube off the mandrel, it must not fight against the rotational motion of the tube (as imparted by the mandrel). Rotational speed change or slippage between the tube and the mandrel may lead to uneven weight and dimension in the tubular cartridges, and such quality issues are difficult to detect and correct during production.
h. The puller device should maintain a firm and constant grip on the rotating tube, even as the latter's surface often has varied and changing properties including tube diameter, coefficient of friction, hardness, material's oily content, average fiber diameter, fuzziness, moisture, spray coatings, out-of-roundness, compaction treatment on tube wall, and the like. With conventional puller devices, tightening the grip is the only method of accommodating differences in nonwoven properties, which may interfere with the tube's rotational motion. Rotational retardation or slip, detectable or not, violates requirements “f” and “g” above.
i. To meet so many requirements simultaneously, the puller device's operation and control should be able to utilize modern automation technology to reduce manual labor and avoid errors and inaccuracy.
Currently, there are several types of puller devices in commercial use, which are widely apart in concept and design but none of which can be considered as satisfactory. The existing types of puller devices include devices with a rotating screw inside the tubular nonwoven; devices with rotating screws on the outside of the tubular nonwoven; devices with multiple canted rolls; devices with gears and pulling arms; and devices with canted rolls with detents, each of which is discussed below.
FIGS. 2 and 3 illustrate a tubular nonwoven system 200 having a first type of puller device 2 (“Type A”), which includes a rotating screw 207 inside a tubular nonwoven 204. Such a device is illustrated, for example, in U.S. Pat. No. 5,366,576; U.S. Pat. No. 5,409,642; and U.S. Pat. No. 5,672,232.
A rotatable mandrel 202 functions as a collector surface for a melt-blowing die 201, which deposits molten fibers 203 onto the surface thereof. The rotatable mandrel 202 is driven at a first rotational speed Ω1 by a first motor “M”. The first motor is connected to a first pulley 214 that is connected by a drive belt 213 to a second pulley 212, which is attached to the mandrel 202.
The rotatable screw 207 (having a slightly larger diameter than the outer diameter of the mandrel 202) is installed at the end of the mandrel 202. A shaft 208, which is positioned through the hollow mandrel 202, drives the screw 207 at a second rotational speed Ω2. The shaft 208 is turned at speed Ω2 by a third pulley 209, which is connected by a drive belt 211 to a fourth pulley 210. The fourth pulley 210 is operably connected to a second motor “M.”
When the rotational speed Ω2 of the screw 207 is significantly (15% to 25%) faster than the rotational speed Ω1 of the mandrel 202, the screw thread cuts into the inner wall of the nonwoven tube 204 and pushes the tube 204 forward in an axial direction toward the cutting device 205. The cutting device 205 cuts the nonwoven tube 204 into tubular cartridges 206 of a desired length.
Although its mechanical system is complex and expensive, Type A pullers with inside screws (e.g., 2) are one of the most popular puller devices currently in use. One shortcoming associated with these types of puller devices 2 is that the rotating screw 207 cuts grooves into the inner wall of the nonwoven tube 204, and the resulting grooves may function as a continuous flow channel for filtrate and contaminants to escape under pressure. This concern is more serious when the flow pressure is high, the filtration requirement is stringent, or the filtrate is of high value.
Another limitation of Type A puller devices is the necessity to replace the complete set of the mandrel and screw each time a product change demands a different inner diameter. Such a requirement increases equipment and operational costs. All other types (discussed below) require only the mandrel to be replaced.
FIGS. 4 and 5 illustrate a tubular nonwoven system 300 having a second type of puller device 3 (“Type B”), which includes rotating screws 307 on the outside of the tubular nonwoven 304. Such a device is illustrated, for example, in U.S. Pat. No. 5,366,576 and U.S. Pat. No. 5,672,232.
A melt-blowing die 301 deposits molten fibers 303 onto a rotating mandrel 302 to form a continuous tubular nonwoven 304. The puller device 3 includes multiple (usually three) screws 307 that are positioned around and against the outer surface of the tubular nonwoven 304. An endless drive belt 309 engages the screws 307, and a pulley wheel 308 is connected to a motor “M”. A drive belt tensioner 310 may be used to ensure the appropriate tension on the drive belt 309. The puller device 3 advances the nonwoven tube 304 to a cutting device 305, which cuts the tube into individual nonwoven cartridges 306 of a desired length. The surface speed of the screws 307 is faster than that of the nonwoven tube 304, pushing it in an axial direction toward the cutting blade 305.
The three puller screws 307 cut multiple grooves on the outer wall of the nonwoven cartridge 306. Except for detracting from the appearance of the product, the screws 307 result in less harm than the cuts on the inner wall that are produced by the Type A puller (the inner screw type) discussed above. However, this system's hardware and operation are more complicated and difficult to use than Type A.
FIGS. 6 and 7 illustrate a tubular nonwoven system 400 having a third type of puller device 4 (“Type C”), which includes multiple canted rollers 407 that are driven and that are pressed against a newly formed tubular nonwoven 404. Such a device is illustrated, for example, in U.S. Pat. No. 4,112,159; U.S. Pat. No. 4,116,738; and U.S. Pat. No. 5,591,335.
A melt-blowing die 401 deposits molten fibers 403 onto a rotating mandrel 402 to form the continuous tubular nonwoven 404. The rotatable mandrel 402 is driven at a first rotational speed Ω1 by a first motor “M”. The puller device 4 includes multiple (usually three) canted rollers 407 that are positioned around and against the outer surface of the tube 404. The canted rollers 407 are driven by a second motor (not shown) at a second rotational speed Ω2. The nonwoven tube 404 is cut by a cutting device 405 into nonwoven cartridges 406 of a desired length.
By adjusting the angles between the axis of the mandrel and those of the canted rollers and by adjusting the speed differential between the surfaces of the tubular nonwoven and the canted rollers, an axial force component is produced that nudges the tube forward and off the mandrel, while the mandrel, the nonwoven tube, and the rollers are in rotational motion. It has been found that simultaneous adjustment of the angles, rotational speed, and compressive force of the rollers 407 is difficult to achieve and is impractical to automate by modern technology. As a result, although nonwoven manufacturers have used Type C puller devices commercially for the longest time, these manufacturers have found it hard to consistently obtain high product quality; and the off-quality ratio is high.
FIG. 8 illustrates a tubular nonwoven system 500 having a fourth type of puller device 5 (“Type D”), which includes gears 505 with puller arms 506. Such a device is illustrated, for example, in U.S. Pat. No. 4,847,125.
In this system 500, a melt-blowing die 501 deposits molten fibers 503 onto a rotating mandrel 502 to form a continuous tubular nonwoven 504. The rotatable mandrel 502 is driven by a first motor “M”. When the nonwoven tube 504 reaches a desired diameter on the mandrel 502, a puller device engages the nonwoven tube.
The puller device 5 includes two gears 505, which are attached to puller arms 506 and which are positioned around and against the outer surface of the tube 504. The gears 505 are rotatable, but not motor-driven. The puller arms 506 pull the nonwoven tube 504 from the mandrel 502 in an axial direction 507, so that the nonwoven tube 504 may be subsequently cut by a cutting device (not shown) into nonwoven cartridges of a desired length.
The Type D puller device is complicated to operate and, thus, has limited practical utility. Because the puller device itself is located in an area previously used for the cutting device, this system is incapable of working in continuous production.
FIGS. 9 through 11 illustrate a tubular nonwoven system 600 having a fifth type of puller device 6 (“Type E”), which includes a large canted wheel 607 with detents that engage the outer surface of a tubular nonwoven 604. Such a device is illustrated, for example, in U.S. Pat. No. 6,736,274.
As with the previous melt-blowing systems, a melt-blowing die 601 deposits molten fibers 603 onto a rotating mandrel 602 to form a continuous nonwoven tube 604. The rotatable mandrel 602 is driven by a first motor “M”. The canted wheel 607, which is driven by a second motor “M”, includes multiple sharp detents on its outer periphery. The detents penetrate into the nonwoven tube 604 to pull the tube 604 from the mandrel 602 to a cutting device 605. The nonwoven tube 604 is cut by the cutting device 605 into nonwoven cartridges 606 of a desired length.
When the detents pierce the outer surface of the nonwoven tube 604, they pull the tube 604 in the direction of the rotational motion of the canted wheel 607. As a result, the detents leave permanent holes 608 in the outer surface of the nonwoven tube 604, which can impact the functionality of the nonwoven cartridge 606. Additionally, the pulling force of the canted wheel 607 may unwittingly cause rotational slippage between the nonwoven tube 604 and the mandrel 602.
Like the Type C puller device, the Type E puller device requires simultaneous adjustments to the speed, the slant angle, and the compression force of the canted wheel 607. Such multiple adjustments are difficult and imprecise. For this reason and because of the detent damage to the resulting product, these Type E systems have limited commercial applicability.
With these conventional puller devices, it has been observed that the forces of pull are imprecise and insufficient. When the puller devices engage the rotating nonwoven tube, they resist the tube's rotational motion with an undesirable and immeasurable torque, which often leads to inconsistency in product quality.
By using the performance criteria (a to i) listed above, the relative merits of each of the said types (A to E) are estimated below in TABLE 1. The requirements are rated on a scale of 1 to 10, where 1 is the least satisfactory and 10 is the most satisfactory.
TABLE 1Performance characteristics of prior puller systemsManufacturing Requirements (rated on scale of 1-10)1 = least satisfactory, 10 = most satisfactoryPuller TypeabcdefghiA (inner screw)3103426458B (outer screws)4104436457C (canted rollers)2104354433D (gears and arms)5 15436753E (canted roll with 5106418385detents)
As observed, none of the puller types scores highly in all requirements. From this background, it is clear that there remains a need for a better puller device and method.